1. Field of the Invention
The subject invention generally pertains to vertically movable doors and more specifically to a door that is particularly suitable for use at a truck loading dock or other location where ventilation, safety or security may be important.
2. Description of Related Art
Many buildings may have a doorway with a loading dock to facilitate transferring cargo between a truck and the building. A loading dock is a platform that is generally at the same elevation as the bed of the truck or its trailer. The dock may also include a dock leveler, which is a vertically movable ramp that compensates for a height differential that may exist between the platform and the truck bed. Dock levelers may also provide a bridge across which personnel and material handling equipment can travel between the platform and the truck.
For protection against weather and theft, the doorway of the building may include a manual or power operated door. Doors for loading docks usually open and close by moving vertically so as not to interfere with the rear of the truck or interfere with cargo and activity just inside the doorway.
When there is no truck at the dock and the weather is mild, the door may be left open to help ventilate the building with fresh outside air. Leaving the door open, however, reduces the building's security and increases the risk of personnel or items inside the building from accidentally falling off the edge of the dock's platform and through the open doorway to the driveway. But even with the door closed, heavy material handling equipment, such a forklift truck, may have enough power or momentum to accidentally break through the door and still fall off the edge.
Barriers of various types are used in a loading dock environment to prevent the accidental run-off noted above. In fact, some loading dock levelers feature extended lips that can provide a run-off barrier when the leveler is in an inoperative position, but these barrier-style levelers do not protect the loading dock door from impact when the door is closed because the extended lip is outside of the door. Examples of barrier-style dock levelers can be seen in U.S. Pat. Nos. 4,920,598 and 5,040,258. Other barriers, such as a simple safety gate better protect the loading dock door, but they are typically manually activated or require a separate operational step to position the barrier. Examples of a gate-type barrier are the Rite-Hite Dok Guardian product and the inventions disclosed in U.S. Pat. Nos. 5,459,963 and 5,564,238.
A variety of other patents are directed to loading dock door systems. U.S. Pat. No. 5,408,789, for example, discloses a unique loading dock door system that automatically places a barrier to both prevent run-off and protect the door, itself, from impact. The patented system may not only include what appears to be a conventional vertically moving door, but also an additional screen door for ventilation and security. For run-off protection and to protect the screen door from impact, a safety barrier (which appears to be a rigid bar) is attached to the screen door. A system of this type, in which the barrier is automatically placed when the door is closed, provides the additional convenience of not requiring an operator to perform an additional operation (in the case of a manually positioned barrier) or an additional driving mechanism (in the case of an automatically positioned barrier) to position the barrier. Further, the system ensures that the barrier is always in place when the door is closed, thus ensuring protection of the door from impact damage. However, because the barrier travels with the door, it is also removed when the door is opened, leaving no run-off protection. Further, the system actually includes two doors with two sets of tracks, which is likely more expensive than a single door. The two doors also take up more floor space in a loading dock area where floor space is often limited. The rigid bar disclosed in this system would also be subject to permanent deformation when impacted by a fork truck or similar conveyance.
In another attempt to provide ventilation to a sectional door, the system disclosed in U.S. Pat. No. 6,092,580, includes a screened gate that can be selectively attached or removed from the lowermost panel of a garage door. Because the screened gate is not part of the garage door itself, it does not have its own rollers for traveling along the door's tracks. Instead, the gate is either attached to the frame of the doorway using Velcro strips (FIG. 6), or the gate stows against the inside face of the lowest door panel (FIG. 11). The screened gate can also be manually detached from the door and left on the ground (attached to the doorway frame with Velcro) to provide a barrier for pets and children. Although it may be an effective system for its intended purpose of providing a barrier to pets and children, while also providing them with ventilation, such a system would be unacceptable for use at a loading dock for several reasons.
First, an industrial barrier for impeding forklifts at a loading dock generally needs to be relatively strong, particularly at floor level where the forklifts travel. With the '580 system, however, the screen, which appears to be one of the weaker members of the door, is placed at the very bottom of the door to serve as a barrier, while the more solid door panels are higher up.
Second, vertically moving sectional doors (e.g., garage doors) typically have a spring-loaded system for counterbalancing the weight of the door panels, thereby making the door easier to operate. Adding or removing weight from the door by selectively attaching or disconnecting a panel can change the weight equilibrium of the door. Depending on whether weight is added or removed, the door may have a strong bias to open or close. This may not be a problem for the '580 system, because the screened gate appears to be relatively lightweight, but a much heavier panel is needed to stop a forklift. Adding or removing the weight of a heavy, truck-stopping panel from an industrial door may cause the door to fling open or close abruptly.